The current hypothesis of contraction of skeletal muscle is that the binding of a "fuel" molecule (ATP) to an active site of myosin induces a local conformational change in the catalytic domain --an enzymatically active part of a molecule. This change is mechanically amplified and leads to a major rotation of the regulatory domain -- a long part at the end of myosin that is enzymatically inert. The rotation of the regulatory domain results in the generation of force and movement. The rotation is coupled to the chemical events occurring at the active site of myosin. The aim of this proposal is to test this hypothesis in a single cross-bridge of contracting muscle fiber. A confocal microscope is modified to allow measurements from a small population (approximately 10) of cross-bridges. The rotation of the regulatory domain and the enzymatic activity are measured simultaneously. The rotation is studied by measuring the anisotropy of fluorescence of probes placed at strategic positions within the regulatory domain. The anisotropy is measured either during transient contraction (created by suddenly releasing ATP from a cage) or during steady-state contraction (by using correlation spectroscopy method). The enzymatic activity is measured by fluorescence of phosphate binding protein excited by light emerging from a Near-Field probe. The anisotropy and enzymatic signals are cross-correlated to establish their causal relationship. The significance of this work is that the prevailing hypothesis will be tested, for the first time in a single cross-bridge of working muscle. This is expected to provide definitive answers about the mechanism of contraction of skeletal muscle.